The quality of radiographic images obtainable at a given X-ray intensity level is limited by noise and the spatial resolution characteristics of the X-ray detector. For example, where an intensifying screen and film combination is used as the detector, an increase in sensitivity of the film with a proportional decrease of the X-ray exposure will result in a grainy image because of quantum mottle, while an increase in the sensitivity of the screen by increasing its thickness will result in a decrease in spatial resolution. If the X-ray target object heavily attenuates the X-ray flux, the image quality can potentially be improved either by increasing the flux intensity or lengthening the period of exposure. Larger clinical X-ray machines thus are capable of providing a variety of exposure levels to adjust to the attenuation of the portion of the body being X-rayed. This increased X-ray flux intensity is obtained by an increase in the X-ray tube current and a corresponding increase in the power drawn from the power supply.
While fixed clinical X-ray machines are provided with power from high capacity electrical power lines that are capable of delivering large surges of current, a portable X-ray unit cannot make such heavy power demands since it must be capable of operating from a normal 115 volt AC outlet or, in the field, from a storage battery or portable generator. Thus, to improve image quality in portable machines, radiographs may be taken over a longer exposure time to yield the same total X-ray exposure that a clinical X-ray machine could provide at higher intensity levels. With typical portable machines, an exposure time of 0.25 seconds or more may be required to complete an adequate exposure of a normal chest, and several seconds may be required to form a satisfactory image of a heavy abdomen. These long exposure times often result in a blurred image because of the normal motions of the body from breathing, heart beats, and voluntary muscle action. In addition, a small portable machine relying on a limited power supply may have to be operated at high values of kVp to provide adequate penetration of the X-ray target, resulting in a sacrifice of contrast.
Presently available mobile X-ray systems are of four types. One common small, lower power unit operates directly from 115 VAC, 60 Hz line power and is usually limited to about 20 mA maximum tube current. A second type of system utilizes 60 Hz, single phase line power, but at 220 VAC, allowing power lines surges up to 100 amperes and tube currents of about 200 to 250 mA maximum. A third type of portable unit utilizes capacitors to store energy for discharges in a manner similar to a capacitor discharge photoflash gun. These capacitive storage units typically are limited to 17 mAs equivalent tube charge at 100 kVp. A fourth type of portable unit utilizes batteries to provide the surge of power during exposures and may be capable of instantaneous power of about 10 kilowatts and an input tube current of about 100 mA. Because of the weight of the batteries, the battery powered X-ray units tend to be quite heavy, typically ranging in weight between 200 and 400 kilograms.
The presently available mobile X-ray machines provide marginal performance where stop-motion exposures are desired or high power levels are needed for penetration. If good stop-motion images are required, current input requirements rise to levels beyond the capability of normal hospital wiring or of ordinary batteries. For example, operation of a battery powered mobile X-ray machine at 400 mA, 100 kVp would require current from a 90 volt battery in excess of 500 amperes, far beyond the capability of present compact batteries (which typically weigh several hundred pounds). Thus, present mobile X-ray generators generally cannot operate at power levels corresponding to tube currents above about 200 mA for line-powered machines or 100 mA for battery powered machines.
Energy storage flywheels have been proposed as an alternative power source for mobile X-ray units although such machines are not presently commercially available. Examples of such machines are shown in the U.S. Pat. Nos. to Grady, 4,322,623, and Jordan, 4,182,967. The apparent limitations of such proposed flywheel power supplies include the relatively heavy weight of large flywheel, motor, and generator combinations and inadequate adjustability and regulation of the power provided to the X-ray tube.